Introduction
Most sharks never stop swimming for their entire lives. Some species have evolutionary traits that allow them to stay in place. However, most sharks require movement to extract oxygen through their gills. For these sharks, stationary resting is impossible.
How sharks rest is a large mystery to shark science. Behaviors have been observed, but a new study finds a new key to discovering how sharks take power naps. The new study discovers that sharks use ocean currents to take turns resting by ‘surfing’ in a conveyor belt formation.
A research team led by Yannis Papastamatiou from Florida International University studies the nocturnal hunting behavior of grey reef sharks. These sharks, from Fakarava Atoll in French Polynesia, never stop swimming.
Study
Papastamatiou notes that some sharks were swimming against the updraft current in a channel during a day dive. Remarkably, these sharks were still, hardly moving their bodies.
The sharks in front of the group would slip backward as they crept to the front of the channel. Therefore, the current would carry them back to the beginning. Sharks inched forward against the current, get carried back, then inch forward again. The process is similar to a strange sort of conveyor belt.
Using their tools, the team built a biomechanical model to measure the amount of energy used by each shark swimming in the updraft currents. Surfing the updraft allowed the grey reef sharks to conserve at least 15% of normal swimming energy expenditure.
Based on the current understanding of ocean currents, the research team realizes that they may find other shark resting spots. So, using multibeam sonar, the team predicted and mapped where updrafts might occur and used tracking sensors to monitor these spots.
The monitoring data suggests that the sharks choose to hang out in updraft currents during the day. The sharks adapt their position to minimize energy expenditure best. For example, incoming tides have stronger updrafts. Therefore, sharks display the conveyor belt behavior more strongly. The sharks go deeper where the current is a little weaker.
Outgoing tides create more turbulence. During outgoing tides, sharks are more spread out and hang out closer to the surface to avoid the rolling water below.
Conclusions
The behavior studied is similar to birds. Birds use updrafts to stay aloft with minimal energy expenditure. Future research may use this information to locate sharks and other species, such as squid, that need to stay moving to live.
Papastamatiou claims that “this study is a nice demonstration of energy seascapes, a spatial representation of how much energy it costs an animal to move through an environment.”
“Marine environments are a lot more dynamic because of the water currents, which are much less predictable. They can change seasonally, throughout the day, and even minute by minute. Ultimately, the energy seascape helps explain why these animals are in this channel hanging out there during the day. Now we have an answer.”